Irrigation is a common practice in agriculture, particularly in areas that may be otherwise marginal or unsuitable for crop production. Irrigation provides the moisture necessary to sustain plant health during the growth cycle, and typically results in improvements to crop yield or quality. Irrigation can also be useful for controlling the moisture of a seed crop to be harvested, moderating the effects of temperature extremes (hot or cold), and as a means of applying vital nutrients to a living crop. In some instances, crop production is not possible without the water provided through irrigation activities, while in others irrigation may serve primarily as insurance against occasional bouts of dry weather.
The water used in irrigation is typically obtained from either ground water or surface water resources. Wells that are drilled deep into the ground to tap into aquifers are a common source, as are rivers, canals and pipelines from surface water resources. In many instances there are concerns over depletion of public water resources, which can make the water efficiency of irrigation activities a vital concern. Irrigation efficiency can be controlled using a number of different approaches, ranging from minimizing water lost to the atmosphere (using a combination of irrigation timing and/or system configuration such as drop nozzles, drip irrigation or even sub-surface irrigation) to maintaining only the bare minimum amount of soil moisture necessary to maintain plant health.
Maintenance of soil moisture at the desired level may involve the use of soil moisture sensors within the field, or methods for tracking the available water in the soil moisture ‘bank’ through the estimation of draws and deposits of water throughout the course of the growing season. The latter is typically done by tracking the deposits due to rainfall or previous irrigation activities against estimations of water drawn out of the soil profile by the crop and/or lost to direct evaporation from the soil, with the goal of maintaining a steady balance between these competing forces.
The estimation of water drawn out of the soil profile by the crop and/or lost to direct evaporation from the soil is usually done using equations for estimating the evapotranspiration of water from the soil. Evapotranspiration (often denoted as ‘ET’) refers to the summation of evaporation and plant transpiration of water from the soil and into the atmosphere. Evaporation accounts for the conversion of liquid water resident near the soil surface or on the plant itself into a gaseous form. Transpiration is the process by which moisture is carried through plants from roots to small pores on the leaves (called ‘stomata’), where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from inside of plant leaves, and has two main functions: keeping the plant cool, and pumping water (and the minerals contained therein) to the plant leaves in support of photosynthesis.
Plants need to cool themselves, as when temperatures are too high, metabolic functions and growth and/or flowering may slow, stop, or the plant may even die. The heat lost from the plant during evaporation from the plant stomata maintains an acceptable temperature within the plant system. The loss of water through transpiration is regulated by guard cells on the sides of the stomata that effectively open and close the pores. These guard cells are sensitive to light intensity, temperature, wind, relative humidity, as well as the carbon dioxide concentration within the leaves. The guard cells must open access to the stomata for the plant to take in carbon dioxide needed for photosynthesis, but the more they are open, the more plants transpire and lose water. As plants transpire, water is drawn from the soil and up through the plant, causing the soil to become drier. In order to maintain an adequate supply of moisture in the soil, precipitation and/or irrigation is required.
As mentioned previously, tracking the amount of water lost from the soil moisture bank, particularly in the absence of soil moisture sensors and/or in the case of forecasting irrigation needs, is typically accomplished using equations for estimating the evapotranspiration of water. Many such equations and methods exist. Perhaps the most common equation used in modern irrigation scheduling is the Penman-Monteith equation, which is used as a standard by the United Nations Food and Agriculture Organization (FAO). The Penman-Monteith equation relates plant properties to weather conditions (including temperature, humidity, wind, and radiation) to the mass or volume of water lost to evapotranspiration. Since the plant properties vary from one type of plant to the next, and can also vary throughout the growth cycle, it is common to calculate actual evapotranspiration of a crop (ETc) using calculations based on a reference crop (usually a short green crop, such as alfalfa or grass) multiplied by modulating coefficients:ETc=ETr×Kc×Ks 
where ETr is the evapotranspiration rate from the reference crop, Kc is a crop coefficient that varies by crop and growth stage, and Ks is a water stress coefficient that reduces the rate of evapotranspiration as soils become dry. At any given point in a crop's growth cycle, the Kc value for a non-stressed crop is simply the ratio of its actual evapotranspiration rate relative to that of the reference crop. Values of Kc may be near 0 for seedlings and range upward to values in excess of 1.0 for some crops at peak vegetative stage with canopies fully covering the ground. Appropriate Kc coefficients for most common crops, varying throughout the growth cycle, have been measured and published by various organizations. The water stress coefficient, Ks, is set to a value of 1.0 for a crop that is not experiencing any moisture stress, but decreases according to the reduction of evapotranspiration of a crop experiencing increasing moisture stress (or other stressors such as disease, nutrient deficiency, etc.).
While widely applied, these approaches to estimating evapotranspiration have several important limitations. Stomatal resistance (or the plant-integrated equivalent often referred to as ‘canopy resistance’), which represents plant-specific properties in ET calculations, is a measure of the resistance to carbon dioxide entering, or water vapor exiting, through the stomata of a leaf, and is controlled by the leaf through the use of guard cells. The canopy resistances to transpiration are dependent upon many factors, including crop and growth stage, and impact overall evapotranspiration calculation in non-linear ways. Thus, these approaches are substantially simplified representations of the movement and use of water within the soil-plant-atmosphere system.
Such approaches also treat the soil moisture as a simple bank, with draws and deposits, but with no understanding of the distribution or movement of water within this bank relative to the distribution and moisture uptake capacity of the plant roots. The potential loss of moisture from the bank due to sub-surface drainage is ignored, as is the ability of moisture to move upward from below the rooting zone due to capillary or osmotic forces. The timing and magnitudes of water inputs, time-varying plant uptake, and changes in the characteristics of the soil with depth can all lead to substantial variations in soil moisture with depth, and this moisture distribution relative to the root distribution of the growing crop is vitally important to understanding the water needs of the crop.
The soil characteristics and soil moisture content also directly affect the strength of the binding of water molecules to the soil, thereby modulating the ability of the plant to extract moisture from it. For instance, water bonds much more tightly to clay soils (with small pores spaces) than it does to sandy soils (with larger pore spaces), causing the moisture content associated with the ‘wilting point’ of clay soils to be substantially higher (i.e., plants will experience moisture stress at higher moisture contents). While the variability of this binding between soils is represented by Ks in the example ETc calculation above, a single value representative of the properties of the soil at a given location must be selected and used, even though the actual relationship is time-varying and dependent upon root distribution relative to the vertical profile of soil characteristics. Field-specific factors that affect water movement and availability also include factors such as organic matter and the influences of compaction, tillage, and decayed root channels or root worm holes, all of which are substantially dependent upon the nature of the farming practices within a given field.
Because of these limitations, there is value in improving the simulation of crop moisture needs using more sophisticated approaches for modeling these complex systems. Modern land surface models, developed largely in response to the need for understanding the large-scale interactions between the soils, plants and atmosphere as important factors driving weather and climate, offer a potential starting point for a next generation of irrigation scheduling models. The earliest land surface models, often referred to as ‘bucket models’, were substantially similar in nature to the typical models used in scheduling irrigation today. Over time, land surface models have evolved to use sophisticated multi-layer canopy and soil models, with explicit simulation of many of the processes at work in the soil-plant-atmosphere system. Such simulations include, but are not limited to:                simulation of runoff and infiltration of precipitation off of or into the soil profile;        gravitational drainage, vapor diffusion, capillary action, and root uptake of moisture within any number of layers within a soil profile;        aquifer depletion and recharge;        direct exchanges of moisture between the atmosphere and the soil surfaces via evaporation, sublimation, condensation and deposition;        vertical diffusion and conduction of internal energy (heat) into, out of, and within the soil profile;        loss and gain of thermal energy to plants through radiation, conduction, evaporation, sublimation, condensation, and deposition;        moisture accumulation and loss to plant leaf surfaces as a result of intercepted precipitation, condensation, deposition, evaporation and sublimation;        rate of photosynthetic activity; and        plant growth and transpiration, including the impacts of weather and soil conditions on the properties and processes of the vegetation.        
Commonly-used land surface models include the NOAH community land surface model, originally developed jointly by the National Centers for Environmental Prediction (NCEP), the Oregon State University (OSU), the United States Air Force, and the National Weather Service's Office of Hydrology (OH). They also include the VIC model, or Variable Infiltration Capacity model, developed by the University of Washington's Land Surface Hydrology group, and the Mosaic model, developed by the National Aeronautics and Space Administration (NASA). Another example is the CLM model, or Community Land Model, a collaborative project between divisions and groups within the National Centers for Atmospheric Research (NCAR). Numerous other land surface models are available for both research and commercial applications.
Land surface model inputs include soil composition and characteristics, topography, vegetation/canopy characteristics, various relationships and characteristics defining the soil-water-plant relationships, and detailed weather information (including detailed precipitation and radiation information). In an agricultural setting, however, other inputs or factors that may be very specific to a given field may be important, such as farming practices and the impacts they have on soil structure, the presence of artificial drainage systems, the specific crop planted on a field and its growth stage, irrigation activities, etc. A collection of techniques for specifying and tuning such field-specific parameters to enable application of land surface models for irrigation-specific scheduling, recommendations and advisories is the subject of this invention.